Increased Radioresistance in Mice Injected with Urethan One Day before X-Irradiation

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Abstract
Adult female (C57LXA)F1 mice received 2 intraperitoneal injections of aqueous urethan (1 mg/gm of body weight), 1 day apart. Twenty-four hours after the last injection, the mice were exposed to a single acute dose of 250-kv X-rays, in the midlethal or lethal range. The 30-day LD50 for the urethan-treated mice was 970 r; the LD100 was 1010r. Corresponding values for control irradiated mice were 790 r and 840 r, respectively. The radio-protective effect of urethan was not seen when the drug was administered 30 minutes before irradiation, nor when the mice were irradiated 7 days after the last urethan injection. The protective effect could not be duplicated by preirradiation exposure (1 or 2 days prior) to 100 r, 200 r, or 300 r. Nonirradiated mice given 2 daily injections of urethan as above exhibited a marked fall within 24 hours, in the total nucleated cell count of femoral marrow and peripheral blood, accompanied by a definite increase in the myeloid/lymphoid cell ratio of the blood. Results suggest that urethan-induced alterations in marrow cell population are accompanied by an increase in the number of relatively radio-resistant cell types, involved in myelopoiesis.

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